1. Field of the Invention
The invention relates to an apparatus for uniformly subjecting a flow-traversable heatable material to a fluid flow.
The term "flow-traversable heatable material" applies in particular to stacks of plates, rods, tubes, pipes, etc., in which between the individual layers spacers are disposed to keep the individual layers a defined distance from each other.
2. Description of the Prior Art
The uniform heating of such a flow-traversable material is possible in many cases only by forced convection because upon radiation heating the heat transfer at the edge parts is substantially greater than at the centre region or core of the heated material. Since, however, frequently very long through-flow paths occur with convection heating, additionally the flow or stream must be reversed at adequately short intervals of time because with convection heat transfer the heat transfer conditions in a flow vary with the flow path, too.
Consequently, an arrangement must be provided which makes available the volume flow necessary for the flow passage together with the pressure necessary for overcoming the pressure loss and simultaneously permits a mirror inverted reverse of the flow, the so-called "reversing"; it must also be taken into account that said forcing pressure can be relatively high, in particular with relatively long flow paths as occur in plate stacks.
For this purpose, for example, in the drying technology, but also in industrial furnace construction, apparatuses are used in which the flow can be reversed by changing the direction of rotation of the fan. For this, only axial-flow fans are used because only this design permits a reversal of the flow direction on reversal of the direction of rotation.
Equal flow performances for both directions of rotation, however, are obtained only for straight blades and a 45.degree. blade angle. In addition, guide wheels may not be used. Consequently, the pressure coefficient and the efficiency are very unfavourable. Admittedly, DE-PS No. 3,248,760 discloses an axial-flow fan in which the blades automatically adjust themselves in accordance with the direction of rotation; however, this solution cannot be used for high-temperature applications because of the hardly solvable bearing and strength problems.
Because such apparatuses are usually operated at relatively high temperatures the axial-flow fans are as a rule mounted in cantilever manner, i.e. in the vicinity of a wall, and consequently generally a deflection of the flow path is necessary. This function-induced design leads, however, necessarily to different behaviour of the axial-flow fan in its two directions of rotation and thus in the associated flow directions. Consequently, in practice an axial-flow fan used for recovering the flow always delivers a different volume flow in the one direction than the opposite direction, thus a correspondingly different flow through the heatable material also results.
Attempts have admittedly been made to compensate this asymmetry in that the axial-flow fan is driven for different lengths of time in the two directions of rotation. However, the results obtained with this solution are not satisfactory because with the volume flow the heat transfer situation also changes and the corresponding relationship is not linear, i.e. in this manner complete compensation cannot be achieved. Moreover, this leads to an unnecessary increase in the operating time and thus in the production costs.
Finally, a further great disadvantage of an axial-flow fan is that it has a very low pressure coefficient compared with corresponding radial-flow fans.
A further possibility of controlling and alos reversing the flow direction are flaps as disclosed for example in Germann Offenlegungsschriften Nos. 3,442,907, 2,724,021, 3,215,509 and 2,600,724. These flaps are usually pivoted flaps with hingelike mounting. As apparent with such flaps, for example, from Offenlegungsschrift No. 2,600,724 the flow, when the flap is arranged on the fan discharge side, can be deflected in different directions or alternatively with corresponding installation of the flaps in a flow duct a cross-section can be alternately opened or closed.
However, in conjunction with the flaps, advantageous as they are in themselves for flow control, the aforementioned known solutions use only low performance fans such as axial-flow fans (German Offenlegungsschrift No. 2,600,724) or simple radial-flow fans unsuitable for high-temperature use.